[time-nuts] Questions about HP 5370B

Mike Feher mfeher at eozinc.com
Tue Sep 14 15:46:17 UTC 2010


I wrote a long description regarding this phenomenon many moons ago. The
thermal fuses in the HP ovens do fail, mostly just because of aging, and not
because of any problems. The fuse holder contains a wax substance which is
normally hard and keeps the contacts closed. If suddenly the contacts were
to get too hot, the wax would melt, at a predetermined temperature which is
a function of the wax, opening the fuse for good, since after the wax cools
down to be hard again it is too late, the fuse is open. Aging can
unfortunately have a similar effect where the springiness of the contacts
eventually separate due to the breaking down of the wax substance. I have
repaired many HP-105 oscillators as well as the 10811A series types by
simply shorting out the fuse, which is actually typically socketed, with
never any adverse results. In fact, one oscillator from a HP-105 that had
this problem over 20 years ago is still working today with a short in the
place of the fuse. Regards - Mike

Mike B. Feher, N4FS
89 Arnold Blvd.
Howell, NJ, 07731
732-886-5960


  

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Dan Rae
Sent: Tuesday, September 14, 2010 11:28 AM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Questions about HP 5370B

  On 9/14/2010 1:23 AM, John Miles wrote:
>
>> As Bruce suggests, you'll want to peek inside to see that you
>> really do have
>> a 10811 oscillator. If so, then it sounds like the thermal fuse
>> (F1, inside
>> the 10811) might be open.
> Just short it out, or if you like, put in an NTE part with a similar temp
> rating.  It serves little or no useful purpose.
>
> -- john, KE5FX
>
>
>
I hate to disagree with John who knows a heck of lot more than I ever 
will, but in this case it will protect the oven from cooking up if the 
control circuit fails with the heater full on, which can happen.

I did have a 5370B with a 10811 that had a bad thermistor in it as well 
as an open fuse.  I'd guess that's why -hp- fitted it.

  But yes, the thermal fuses can and do fail open for no good reason, 
and it sounds like this has happened here, but I would not recommend 
shorting it out permanently.

Dan





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